Wes Brown insists Manchester United will need to tighten up at the back when they tackle Chelsea on Sunday.
The Red Devils preserved their unbeaten home record in Europe in midweek against CSKA Moscow, but conceded three times in the draw.
Sir Alex Ferguson's champions tackle the league leaders at Stamford Bridge on Sunday knowing they must be at their best to avoid defeat.
Chelsea have been in superb form in the league at home this season with a 100 per cent record and have conceded just once while scoring 15 times.
Brown admits United were not at their best defensively against CSKA and that taking on the Blues will be their toughest test of the season.
"We cannot concede like that when we go to Stamford Bridge," explained Brown.
"We have to be tighter at the back and give the strikers the chance to do a bit of damage at the other end.
"It will be our toughest game of the season. They've been doing very well. They are a well organised team and they will be very strong.
"They are scoring goals at the moment and we will have to be really on top form to think of getting any points."